Section 8.3 Bond Properties

Section 8.3Bond Properties

Bond Properties

In this section…

a. Bond orderb. Bond lengthc. Bond energyd. Bond energy and H

Bond Order = # bonds between two atoms

Bond Length = distance between nuclei of bonded atoms

Bond Length Trends: Larger atoms make longer bonds

Bond Length Trends: Multiple bonds are shorter than single bonds

Bond Energy: Energy required to break one mole of bonds

O-H bond energy = 463 kJ/mol

Bond energies range from about 150 kJ/mol to about 950 kJ/mol

Single bonds are from 150 – 500 kJ/mol

Bond Energy Trends: Larger atoms make weaker bonds

Bond Energy Trends: Multiple bonds are stronger than single bonds

Resonance Effects on Bond Properties

number of bonding pairsBond order = number of bond locations

Bond length and energy follow same trends:

Using Bond Energy to Calculate Enthalpy Change

Breaking bonds requires energy: endothermic

Forming bonds releases energy: exothermic

rxnH = energy of bond broken energy of bonds formed

NOTE: This only works for gas-phase reactions.

Using Bond Energy to Calculate Enthalpy Change

rxnH = energy of bond broken energy of bonds formed

Bond Energies, kJ/molH-H 436N-H 391N≡N 945

Section 8.4Charge Distribution in Molecules

Bond Properties

In this section…

a. Electronegativity and Bond polarityb. Formal chargec. Resonance structuresd. Partial charge

Bond Polarity

Electronegativity

Continuum from Covalent to Ionic Bonding: Bond Character

The larger the electronegativitydifference, the more polar thebond.

More polar = more ionic character.Less polar = more covalent character.

Electron Distribution in Molecules

Formal Charge: Assume all bonding electrons shared evenly between bonded atoms

Formal Charge = Group # - # lone pair electrons – ½ # bonding electrons

Formal Charge = Group # - # assigned electrons

Using Formal Charge to Rank Nonequivalent Resonance Structures

Rule 1: avoid large or split chargesRule 2: negative charges are more stable on more electronegative atoms

Electron Distribution in Molecules: Partial Charges

Actual atoms in molecules have partial charges.

Oxidation Number -2 +4 -2 -2 +4 -3Formal Charge 0 0 0 -1 0 0